High frequency radio circuits



NOV. 17, 1931. R L, DAVls` 1,832,621

' HIGH FREQUENCY RADIO CIRCUITS Filed March 27, 192e lllllllllll'l'ill'l' Illi'lllI-J INVENTOR l ,QoberfLDol/l'.

ATTORNEY Patented Nov. 17, 1931 UNITED STATES PATENT OFFICE ROBERT Il. DAVIS, F PITTSBURGH, PENNSYLVANIA, ASSIGNOR T0 WESTINGHO'USE ELECTRIC & MANUFACTURING COMPANY, CORPORATICN OF PENNSYLVANIAV l HIGH FREQUENCY RADIO CIRCUITS Application filed March V27, 1928. Sera1 No. 265,020.

Myl invention relates to amplifiers and particularly to amplifiers for use with very high frequencies.

It is customary to so arrange vacuumtube amplifiers that a high direct-current potential must be impressed upon the plates. If the amplifiers are of the high-powertype, requiring that cooling Water be supplied to the plates, it has heretofore been necessary to supply the water through a long rubber hose, or other insulating pipe, in order that the resistance of the long column of Water shall preventleakage of the directcurrent, and of the alternating current, to ground. l

Partly, in order to prevent the passage of radio-frequency current and partlyf to save space, the long hose is usually in the form of a coil( "When high frequencies are employed, the distributed capacity of the coiled hose filled with water is sufficient to interfere with effective insulation of the radio-frequency potential from the ground.

My invention avoids this difficulty by so rearranging the amplifier circuits that the anodes shall be nearly at ground potential, as regards radio frequency. For this pur pose, it is necessary that 'the cathodes be at a high radio-frequency potential relative to the ground. This requires that the cathodes be separated from the cathode-heating supply means by choke coils.

It is an object of my invention to provide an oscillating circuit in which the vinductance of said choke coils shall serve as the tuning inductance.

It is a further object of my invention to provide an arrangement in which the inductance in the output circuit shall comprise two inductors in parallel and the totalinductance be thereby reduced.

It is a further'object of my invention to provide an arrangement in which the interelectrode capacity of the tube and the inductance of the choke coils shall constitute elements of the tuned circuit. Y

It is a further object of my invention to arrange a push-pull amplifier' in such a way that the anodes shall be connected to a point of ground potential in the input circuit and the cathodes shall be separate from the input circuit.

Other objects of my invention and details of the construction willbe readily understood from the following detailed description and the accompanying drawing, in which the single figure is a diagram of the circuits and apparatus.

In the drawing, the input circuit is composedof an inductor 1 and a tuning capacity comprising two condensers 2, mechanically connected to be adjusted simultaneously. The point between these two condensers may be connected'to ground or to a source of extra signalling current, if desired.

-The midpoint of the inductor 1 is'connected through a register 3 to a ground connection 4. The connection includes a` C-battery 5 and a condenser" 6 in parallel. The condenser is of sufficient capacity to actas a connection of substantially Zero impedance for the frequency to which the circuit 1-2 is tuned. From two adjustable points near the extremities of the inductor 1, connections 7 and 8 extend to the grids 9 and 10 of two amplifier tubes 11 and 12, the anodes 13 and 14 of these tubes are connected together by a conductor 15 and fromthis conductor through a stopping condenser 16 and a damping resistor 17 to the ground connection 4 and to the negative end of a B-battery 18. The positive end of the B-battery is connected through a radio frequency choke 19 to the vconductor 15. The condenser 16 pre vents the Vshort circuit of the battery 18, but is large enough to offer no substantial impedance to the high-frequency currents in the tubes. The resistor 17 is of sufficient magnitude to prevent any Asubstantial flow of currents of parasitic frequencies.

The cathodes 20 and 21 of the two tubes are illustrated as ordinary filaments. If desired, ilamentary or other heating members may be employed, distinct from the cathodes themselves. If this is done, lthe cathode proper should be connected to the midpoint 22, in the case of the tubes 11 and 23, in the case of the Vtube 12. These midpoints are established by connecting a pair of condensers across the cathode-heating circuit, that is,

transformer is grounded as indicated at 31 and the terminals of the secondary are connected, respectively, to midpoints of two inductors 32 and 33.

The inductor 32 is connected in a lead extending from one terminal of the filament 20 to one terminal of the filament 21 and the inductor 33 is similarly connected from the other terminal of the filament 20 to the other terminal of the la-ment 21. The inductors 32 and 33 in parallel constitute the tuning inductance as explained below. It is not necessary that they have as great inductance vas would be required if they were to act as aI choke coil to prevent the high-frequency potential present upon the cathodes 20 and 2l from passing to the transformer-secondary 30 and thence to ground.

The midpoints of the inductors 32 and 33 are connected also to a pair of condensers establishing a midpoint 34 which is connected to the grounded terminal of the resistor 17.

Output leads 36 are connected from two separated points upon either the inductor 32 or the inductor 33.

A neutralizing condenser 37 is connected lbetween the midpoint 23 associated with the tube 12 and the grid 9 of the other tube 11.

It is thus connected to the conductor 7 and so to that end of the inductor 1 opposite the connection to the grid 10. Similarly, a neutralizing' condenser 38 is connected between the midpoints 22 and the grid 10 and thus, through the conductor 8, to that end of the inductor 1 opposite the connection to the grid 9.

In the event that the'inductance in the inductors 32 and 33 must be greater than is desired for tuning purposes, in orderv to effect the necessary separation, at radio frequency, between the transformer 29 and the filaments. these inductors may be made smallver thanthis requirement indicates and supplemented by inductors 40 and 41 between the midpoints of the i'nductors 32 and 33 and the terminals of the secondary 30. The supplemental inductors may, if desired, be retained even when the tuning inductors are large.

In Ythe operation of the device, direct-current potential is impressed upon the anodes 13 and 14 from the battery 18. The directcurrent circuit is completed from the cathodes 20 and 21 to the midpoints of the inductors 32 and 33 and thence to the ground at 31, returning to the battery 18 from the ground connection 4.

In order that direct current shall pass over this circuit, the cathodes must be heated. lf the cathodes are the filaments themselves, they are heated by the current from the transformer 29 over a circuit through inductor 41 and the two halves of the inductor 32 in parallel to the two filaments, returning from the filaments over the two halves of inductor to the midpoint thereof and from thence through the inductor 40 to the opposite terminal of the secondary 30. lf the filament-s 2O and 21 are not themselves the cathodes, the circuit just traced conveys heating current to them and they in turn heat the cathodes.

The potentials of the grids 9 and 10 are varied simultaneously and in opposite phase by the current in the resonant circuit 1-2 Direct-current potential is impressed on both grids alike by the battery 5. The highcapacity condenser 6 ensures that the radiofrequency potential of the midpoint of inductor 1 is fixed and the resistor 3 prevents the development of parasitic oscillations.

The plates 13 and 14, being maintained at a xed potential as regards the radio frequency by the connection extending through conductor 15, condenser 16 and resistor 17 to the ground connection 4, are at ground potential as regards radio frequency. Any arrangement for cooling the tubes need only provide against leakage of direct current. Water supplied throi'lgh a coiled hose is, therefore, satisfactory for cooling the plates even when the very highest frequencies are contemplated.

rlChe resistor 17 acts to prevent the development of parasitic oscillations and also conducts to `ground any small high-frequency currents which result from the capacity within one tube not exactly matching that within the other.

Since the plates are at a fixed potential, the difference in potential between plates and filaments, which results from the alternatingcurrent potential impressed upon the grids, will be manifested as radio-frequency potential. changes upon the cathodes. These potential changes Will be greater than the potential changes upon the grids because of the ordinary amplifying action of the tubes. Amplified radio-frequency current will. thus be Set up in the output circuits of the device.

For the tube 11, the output circuit extends through the inter-electrode capacity of the tube to the cathode 20, thence through the upper halves of inductors 32 and in parallel and through the centering condensers to the point 34, thence over the resistor 17 and through the large condenser 16 to the plate 13. A similar circuit through the inter-electrode capacity of the tube 12 may be traced over the two lower halves of the inductors 32 and 33 in parallel to the point 34 and so to the plate 14.

It is not necessary to consider the two output circuits traced above separately. Gne output circuit for the two tubes may be traced, including all of inductor 32 in parallel with inductor 33 as the inductance and the inter-electrode capacity of tube 11 in series with that of tube 12 as the capacity. Since in this circuit the inductance is twice and the capacity half that for the circuit first considered, the frequency is the same.

Whether the output be considered as two tuned circuits or one, it is a parallel-resonant circuit in either case and therefore acts to prevent radio-frequency currents reaching the transformer. The inductors 32 and 33 therefore need not be designed as if they were to act as choke coils.

Because ,the potentials upon the `grids 9 and 10 are in opposite phase, the currents set up in the two output circuits just traced produce additive effects in the output connections 36. The radio-frequency current or potential introduced at the input results, therefore, in amplified radio-frequency Current or potential in the output.

The action of the resistor 17 in the two oscillating circuits just traced is sufficient to prevent the circuits from oscillating at parasitic frequencies.

Further prevention of undesired oscillations is effected by the neutralizing condensers 37 and 38. The potential chang-es on the cathode 21 may be regarded as the output of a generator between the plate 14 and the cathode. The condenser 37 establishes a connection from the cathode 21, that is, from one terminal of said generator, to the conductor 7 and so to the upper part of the inductor 1. The current in this connection flows through the upper part of the inductor 1, the resistor 3, condenser 6, resistor 17 and condenser 16 to the plate 14, which is the other terminal of said generator.

The inductor 1, acting as an auto-transformer, impresses corresponding but opposite potentials upon the conductor 8. The potential thus impressed upon the grid 10 through the condenser 37 and over the conductor 8 is` therefore, opposite in phase to the potential impressed upon the grid l0 through the inter-electrode capacity between the cathode 21 and the arid 10. Consequently, by properly adjusting the condenser 37, the tendency of the tube 12 to produce selfoscillation may be neutralized. In the same way, the condenser 38 may prevent the spontaneous generation of oscillation by the tube l1.

The inter-electrode capacity, which is in series with the upper halves of the inductors 32 and 33 in parallel, consists of the capacity between the cathode and the grid, in series with the capacity between the grid and the, plate, plus the capacity between the filament and the plate directly. The resultant of these capacities, with the inductance resulting from the parallel connection, determines the frequency of the tuned output circuit. If this frequency is too high, the capacity within the tube may be supplemented by the capacity of the adjustable condenser 24. Similarly, the capacity within the tube 12 may be supplemented by the adjustable capacity oftheV condenser 25. By means of these adjustable theinductance in one of thetuned circuits.A may be further diminished by so placing the coils that the mutual inductance between them tends to diminish the combined inductance of the two in parallel. That is, one

coil should be so placed that the fiuX estab- ,9,5

lished thereby shall act. not additively,but subtractively upon the flux in the other coil established by the current from the common lead.

In the usual connection for amplifiers, theI capacity which must be neutralized to prevent re-generation is the capacity between the grid and the plate. In the connection here described, the capacity which must be neutralized to prevent re-generation is the capacity between the grid and the filament.`

This is much smaller than the capacity between the grid and the plate. The diiiiculty in neutralizingr the tendency to re-generation due to capacity within the tube is, therefore less with the arrangement here described.

While I have described but one specific embodiment of my invention and have mentioned only a few modifications in the details thereof, many other modifications of the details and many variati-ons in the application of the Vprinciple of my invention will be obvious to those skilled in the art. I, therefore,

' do not intend to be limited except as required by the prior art or indicated in the accompanying' claims.

I claim as my invention:

1. In an amplifier, an input inductor, a vacuum tube and a tuned output circuit, said input indnctor constituting part of the grid- -plate circuit and said output circuit including the interelectrode capacity of the tube in series with inductance, only a portion of said inductance being employed to determine the resonant frequency of said output circuit.

2. In a push-pull amplifier, an input in-l 85 If, then, the frequency is still y ductor, a pair of vacuum tubes, a connection substantially devoid of inductance connecting the plates of said tubes together and to an intermediate point of said inductor.

3. In a push-pull amplifier, an input inductor, a pair of vacuum tubes, a connection substantially devoid of inductance connecting the plates of said tubes together and to an intermediate point of said inductor and connections from points of said inductor on opposite sides of said intermediate point to the grids of the respective tubes.

il. In a push-pull amplilier, an input inductor, a pair of vacuum tubes, a connection substantially devoid of inductance connecting the plates of said tubes together and to an intermediate point of said inductor, connections from points of said inductor on opposite sides of said intermediate point to the grids of the respective tubes and neutralizing condensers connected between each filament and the grid of the other tube.

5. In a vacuum-tube cathode-heating system, a cathode-heating means for each tube, leads connecting the terminals of one cathode-heating means to those of another, an inductor in each lead, a source of heating current and connections from said source to said cathode-heating means through said inductors, and means for separating the radiofrequency component from the direct-current component of the current iiowing through said inductors.

6. In av vacuum-tube cathode-heating system, a cathode-heating means Jfor each tube, leads connecting the terminals of one cathodeheating means to those of another, an inductor in each lead, a source of heating current and connections from said source to an intermediate point of each inductor, whereby the cathode-heating devices are connected in parallel to the source of heating current, and a capacity connected on one side t0 the terminals of a cathode-heating means in parallel and on the other side to intermediate points of the respective inductors in parallel, whereby said capacity and the parallel-combined inductance of portions of two of said inductors constitute a tuned circuit.

7. In a push-pull amplifier, an input circuit, a source ot' heating current, two vacuum tubes having their anodes connected together and to the input circuit, cathode heating means for each vacuum tube means including inductors for connecting said cathode heating means together and to the source of heating current said means also connecting the cathode together through said inductors in parallel.

In testimony whereof, I have hereunto subscribed my name this 22nd day of March,

ROBERT L. DAVIS. 

